The present invention relates to a marine riser system and method of installation. In particular, it relates to a method and apparatus for connecting flexible flowlines between a submerged fixed-position riser section for connecting a surface facility to a subsea wellhead or manifold system.
In the production of fluid hydrocarbons from deepwater marine oil and gas deposits, a fluid communication system from the marine bottom to the surface after production is required. Such a system, commonly called a production riser, usually incudes multiple conduits through which various produced fluids are transported to and from the surface, including oil and gas production lines, service and hydraulic control lines and electrical imbilicals.
In offshore production, a floating facility can be used as a production and/or storage platform. Since the facility is constantly exposed to surface and sub-surface conditions, it undergoes a variety of movements. In such a zone of turbulence, heave, roll, pitch, drift, etc., may be caused by surface and near surface conditions. In order for a production riser system to function adequately with such a facility, it must be sufficiently compliant to compensate for such movements over long periods of operation without failure.
One example of such a marine riser is the compliant riser system disclosed in U.S. Pat. No. 4,182,584. This compliant riser system includes (1) a lower section which extends from the marine bottom to a vertically-fixed position just below the zone of turbulence that exists near the surface of the water, and (2) a flexible section which is comprised of flexible flowlines that extend from the top of the rigid section, through the turbulent zone, to a floating vessel on the surface. A submerged buoy is attached to the top of the rigid section to maintain the rigid section in a substantially vertical position within the water. With riser systems of this type difficulties could arise in installing and maintaining the flexible conduits. The flexible flowline is attached to a rigid section such that the end portion adjacent the fixed or rigid portion is not attached at a normal catenary departure angle. This can result in localized stresses, causing undue wear in the flexible flowline at its terminal hardware. If a natural catenary shape is assumed by the flowline, it approaches the fixed position section pointed upwardly, nearly vertical at its point of suspension.
It is an object of this invention to provide a compliant riser system in which the flexible section assumes a substantially vertical departure angle at its terminal portion, whereby the flexible section conduits are supported longitudinally with relatively low transverse force vectors. It is another object to provide a unique yoke assembly for connecting a fexible flowline bundle to a submerged riser support. It is a further object of this invention to provide a method for connecting an ocean floor base (e.g., a subsea wellhead or the like) to a marine surface facility through a compliant riser having a buoyed lower riser section extending from the marine bottom toward the surface facility and terminating at a predetermined vertical position below turbulent water. This can be achieved with a releasably mounted yoke assembly which provides terminal support at one end of a catenary flowline during installation.